The present invention relates to a joining method and structure of metal members, in which a first metal member and a second metal member, which have different coefficient of thermal expansion from one another, are overlapped and joined.
A structure using steel plate members and aluminum plates has been widely applied to a vehicle body of an automotive vehicle and so on for the purpose of weight reduction in particular. Herein, a mechanical joining, such as a blind rivet, self-piercing rivet, or clinching, has been used as means for joining the steel plate and the aluminum plate. For example, U.S. Patent Application Publication No. 2005/0140158 A1 discloses a joining structure of vehicle-body members in which a joint portion of an aluminum roof panel and joint portions of side, front, rear roof rails are joined by a self-piercing rivet.
Meanwhile, a frictional spot joining for joining the first and second metal members, which have different coefficient of thermal expansion from one another, with a frictional heat has been practically used recently. In this frictional joining, the overlapped first and second metal members are pressed with a rotating tool of a frictional spot joining apparatus from a side of the first metal member, a frictional heat is generated by contact of the rotating tool with the first metal member, the first metal member is softened with the frictional heat generated and made in a plastic flow state, and the overlapped first and second metal members are spot-joined in a solid state (see U.S. Patent Application Publication No. 2005/0035180 A1, for example).
According to the above-described joining structure of vehicle-body members, many rivet joint portions are formed along the joint portions of the aluminum roof panel and the side, front and rear roof rails. Thus, the number of rivets used is considerably large, so joining costs including material cost may increase.
According to the above-described friction spot joining of the first and second metal members having different coefficient of thermal expansion, deformation due to a thermal strain may occur to the member having higher coefficient of thermal expansion because the joining is conduced while the members are in a thermal-expansion state. In particular, in a case where a light-metal member having higher coefficient of thermal expansion and a steel member are spot-joined, there may occur a considerably large deformation due to the thermal strain to the light-metal member.